A sewage structure, known from U.S. Pat. No. 4,373,381 has several hollow concrete parts, the part, which is the upper part in the installed position, accommodating in its test position an air sac as test seal. In this installed position, the test seal is connected over a chain with a sprocket part of the structure. In the pressureless position during the installation of the air sac, the whole of the test device can be shifted into an unwanted inclined position in the hollow concrete part and, under test conditions, with simultaneous action of the test pressure in the interior of the shaft, the danger exists that the test device will become detached in the region of the air sac lying against the wall and, with that, the operating personnel will be exposed to danger.